1. Field of the Invention
This invention relates to test systems and, more particularly, to test circuit boards including built-in resistive devices.
2. Description of the Related Art
Circuit boards come in may different types. One common type of circuit board is a printed circuit board. Printed circuit boards generally have one or more layers of insulating or dielectric material which may be laminated together. Each layer may include multiple signal paths or “signal traces” which are used to propagate signals. In addition, some layers may be used only to provide power or ground and may be considered as a solid plane. These types of layers are typically referred to as power and ground planes, respectively. Further, other layers may include both power or ground planes as well as signal traces.
The circuit board is typically made from an insulating material such as fiberglass or similar dielectric material which may provide isolation between signal traces as well as a rigid backing for mechanical strength.
The signal traces are typically thin metallic “wires” which have been etched from a pattern which has been printed onto a metal layer which may be bonded to the surface of the circuit board. The metal is generally copper or some other similar conductive copper alloy. Depending on the type of process used to manufacture the circuit board, the unused metal may be etched away leaving the signal traces and any other metallic contact surfaces intact.
To connect signals, power and ground between layers, one or more types of “vias” may be used. Generally speaking a via is a hole that is drilled or bored through one or more layers of a circuit board. A metallic finish is plated to the inside surface of the hole, thereby enabling traces or connections on each layer to be connected. Typically, at places where a signal trace is connected to a via on a given layer, a metallic annular ring is formed to which the trace to connected. One type of via is a through-hole via. A through-hole via is typically a hole which is bored completely through all circuit board layers, thereby enabling any layer to be connected to any other layer. Another type of via is a blind via. Blind vias are typically used to connect a surface layer such as a top or bottom layer to an inner layer. Thus, a blind via may be bored through one or more layers but not completely through all layers of a circuit board. Another type of via is a buried via. Buried vias are typically used to connect internal layers of the circuit board. Thus, from either surface of the circuit board, the blind vias may be hidden.
Generally, design rules allow the traces to be routed with as much space between traces as possible. However, depending on such factors as the density of the signal traces and circuit components, signal traces and even vias may necessarily be routed relatively close together.
At high frequencies, the traces on a circuit board may act like transmission lines, thus certain trace characteristics become important when considering signal integrity. Generally, for maximum power transfer and minimum signal reflection it is important to match the impedance of the load to the impedance of the source. Depending on how well these impedances are matched may determine how much the input signal is distorted or reflected as it propagates down the trace.
In many conventional circuit boards, to match the source and load, a termination resistor may be soldered at one or both ends of a signal trace. This may be especially important when analyzing the output signals of a device. An analyzer probe connected to an output pin may cause the signal to be distorted. Accordingly, a termination resistor may be soldered between the output connection and circuit ground, for example. However, depending on the density of the traces and vias on a circuit board, it may be difficult if not impossible to place a resistor on the board. Thus to convey signals at very high frequencies on a conventional circuit board, signals may be distorted due to an impedance mismatch. Therefore, the frequency of operation of a conventional circuit board may be limited.